An elevator installation is installed either inside or outside a building. It essentially consists of a car, which is connected by supporting means with a counterweight or a second car. By means of a drive, which acts selectively on the support means or directly on the car or the counterweight, the car is moved along substantially vertical guide rails. The elevator installation is used to transport persons and goods within the building over one or several floors.
The elevator installation comprises means to protect the elevator car in case of a failure of the drive, the support means or other elevator components. In order to detect such failure, the travel speed of the elevator car is usually monitored. Upon detection of a fault, usually multilevel security measures are initialized. A first security measure is, for example, a shutdown of the elevator drive, or a motor of the drive and actuation of the drive brakes in order to stop the elevator car. This is done for example by interrupting an elevator safety circuit. Another security measure involves the activation of a safety gear or appropriate safety brakes. They can decelerate the elevator car on the guide rails in case of need. Depending on the position of the elevator car or its operating mode, the travel speed is nowadays increasingly monitored by a so-called electronic limiter, which can initialize the necessary safety measures.
From the international application WO 2003/011733 is known a device which, by means of a coded absolute position-measurement system, provides secure information to a controller of an elevator. From a position change as a function of time, a resultant travel speed can be calculated. Since at the same time the position of the elevator car is known, the entire travel course can be regulated and monitored.